1. Field of the Invention
This invention relates to a thermosetting resin composition providing rapid curing and being distinguished in low shrinkage when used according to the resin transfer molding method (which will be referred to as RTM or R-RIM hereafter) which is one of the molding methods for fiber reinforced thermosetting plastics (which will be referred to as FRP hereafter).
2. Description of Related Art
RTM is characteristic in that it permits low pressure, low temperature molding so that the equipment investment such as mold and press costs can be reduced. However, the method has various problems in that its finished product output (productivity) and the mechanical characteristics of the manufactured material (moldability) are poor as compared with those of other conventional methods and also that it provides molded articles whose surface has poor smoothness and therefore it is unsuitable for producing outside plate or shell plate moldings which require mirror surface-like, beautiful appearance.
In order to remedy these shortcomings, there is a need for a resin composition having improved curing performance and more rapid curing at low and moderate temperatures as well as low shrinkage.
Generally, thermosetting resins containing a vinyl monomer as a crosslinking agent have a high volume reduction ratio upon curing, e.g., as high as 5 to 12%. This not only causes decrease in strength, occurrence of cracks, bend or warp, and the like but also deteriorates the surface smoothness of molded articles due to raising of glass fiber contained in the reinforced resin composition used as a starting material on the surface of the molded articles.
In order to overcome the above-described problems, there has been generally used a method in which a thermosetting resin is blended with a thermoplastic resin such as polystyrene or polyvinyl acetate. In order for the thermoplastic resin to effectively act as a low shrinkage agent, the molding temperature upon molding by curing must be high enough. In fact, there has been obtained no sufficient shrinkage lowering effect by methods other than heat-molding at temperatures not lower than 100.degree. C.
That is, the R-RIM methods which involve molding at low or moderate temperatures of lower than 100.degree. C. exhibit only insufficient shrinkage lowering effect. Conventional approaches for the problems have been concentrated on the improvement of the low shrinkage agent. For example, in the method disclosed in Japanese Patent Publication (Kokai) No. 60-141753, excellent shrinkage lowering effect is obtained at 20.degree. C. However, this improvement is achieved at the sacrifice of curing time, i.e., 6 to 8 hours are necessary to cure the resin. Therefore, rapid curing and low shrinkage have not been achieved at the same time by conventional approach.